Jupiter at opposition August 19-20, now at its best
Jupiter at opposition
In 2021, mighty Jupiter will come to opposition on the night of August 19-20. That’s when Earth will fly between the sun and Jupiter, and Jupiter will appear opposite the sun in our sky. Jupiter, always bright, is brightest at or near opposition. You’ll find Jupiter easily now as a blazing light, rising in the east as the sun sinks below the western horizon. Think of us on Earth, sweeping between the sun and Jupiter in our smaller, faster orbit.
Jupiter will come to opposition on August 20 at about 00:00 UTC; translate UTC to your time. In United States time zones, that’s August 19 at 8 p.m. Eastern Daylight Time, 7 p.m. Central Daylight Time, 6 p.m. Mountain Daylight Time, 5 p.m. Pacific Daylight Time, 4 p.m. Alaskan Daylight Time, and 2 p.m. Hawaiian Time. Jupiter will be closest to us only five hours after the exact moment of opposition. Its closest point to Earth will come at about 05:00 UTC on August 20.
The night of opposition is special. But opposition marks the middle of the best time of year to see an outer planet. So start watching Jupiter now. It’ll be near the full moon – a seasonal Blue Moon – around the nights of August 19 to 22, 2021.
Jupiter at opposition near Saturn on sky’s dome
Jupiter at opposition has been doubly exciting in recent years. That’s because Jupiter and Saturn had their great conjunction in 2020. Astronomers use the word conjunction to describe meetings of planets and other objects on our sky’s dome. They use the term great conjunction to describe meetings of Jupiter and Saturn, our solar system’s two biggest worlds. After the 2020 conjunction, these two worlds won’t come to conjunction again until 2040.
As viewed from above the solar system, Jupiter passed between Saturn and the sun on December 21, 2020. In our sky, we’ve seen bright Jupiter near golden Saturn for some years. So it should be no surprise that Saturn’s opposition happened around now, too. In 2021, Saturn reached opposition on on August 1-2.
Both Jupiter and Saturn are in front of the constellation Capricornus the Sea-goat at opposition. Saturn is in Capricornus now and Jupiter crosses into Capricornus (from Aquarius) around August 18. At opposition, Jupiter will mark the eastern border of the Sea-goat. The king of planets will remain in front of Capricornus until mid-December 2021, when it will cross back into Aquarius.
Saturn, on the other hand, moves more slowly than Jupiter on the sky’s dome (because it’s farther from the sun, and so moves more slowly in orbit). In fact, its slow motion in front of the stars caused early stargazers to call Saturn by a name that meant oldest of the old sheep. Saturn will remain in front of Capricornus until February 2023.
At opposition, Jupiter and Venus at once
As the biggest planet in our solar system, Jupiter is always bright. It shines more brightly than any star in the evening sky. But there’s no way to mistake Saturn for Jupiter. Dazzling Jupiter outshines Saturn by some 18 times.
Among starlike objects in our sky, only Venus is brighter than Jupiter. And Venus is up in the west after sunset while Jupiter is in the east in the evening. Venus is nowhere near Jupiter on the sky’s dome. In fact, it’ll be possible around August 19-20 – the time of Jupiter’s opposition – to see Jupiter and Venus in the same sky at dusk. As if they’re sitting on two ends of a seesaw, they’ll appear appear on opposite sides of the sky. Venus will be blazing low in the west, while Jupiter – opposite the sun – will be sitting low in the east at sunset. As Venus descends toward the western horizon, Jupiter will be ascending in the eastern sky. You’ll need an unobstructed horizon in both directions to see both Venus and Jupiter. As dusk subsides into evening, Venus will be descending in the evening sky. Meanwhile, Jupiter will be ascending in the east.
How often does Jupiter reach opposition?
Jupiter comes to opposition roughly every 13 months. That’s how long Earth takes to travel once around the sun relative to Jupiter. As a result – according to our earthly calendars – Jupiter’s opposition comes about a month later each year.
Two years ago – 2019 – Jupiter’s opposition date was June 10.
Last year – 2020 – it was July 14
This year – 2021 – it’ll be August 12-20
Next year – 2022 – it’ll be September 26.
Jupiter, the failed star
Jupiter isn’t a rocky planet like Earth. It’s more like a failed star, not massive enough or hot enough inside to spark thermonuclear fusion reactions, but some 2 1/2 times more massive than all the other planets in our solar system combined. For Jupiter to shine as stars do, you’d need some 80 Jupiters – rolled into a ball – to be hot enough inside to spark thermonuclear reactions.
Yet on this August night – as Jupiter rises opposite the sun – you can imagine what our Earth would be like if Jupiter did have enough mass to shine as stars do!
Bottom line: Look for Jupiter on the night of August 19-20, 2021, as this world comes to opposition, the point opposite the sun in our sky.